Saturday, October 31, 2009

Spaceship Design 102: Life Support

Growing Algae
Human life support is complicated, bulky, and it smells bad. If we only sent robots we would not have to mess with it. But if we go in person we have to deal with it. First, go to the life support page at Atomic Rockets.

To begin with we will need a cabin. Here there is a big difference between short missions, up to a day or two, and longer ones. Short term passengers can sit in airliner style seats, crew at their work stations, and the galley needn't be much more than a refrigerator and microwave oven. But as missions get longer you need bunk space for off duty crew, and at some point cabins or bunkrooms and a real galley.

From a comparison of railroad sleeping cars versus coaches, sleeping accommodations take up about 10 cubic meters per person, 2-4 times as much room as airliner style seating. Add a galley and dining compartment, storage space, and some this and that brings the minimum requirement to around 15 cubic meters per person, give or take.

(The ISS is much roomier, Wikipedia claiming a 'living volume' of 358 cubic meters for a crew of six, or nearly 60 m3 per person. But I don't know how living volume is defined, for example whether it includes working spaces. Total pressurized volume of the ISS is reported as about 1000 m3.)

Man does not live by bread alone, but along with water and oxygen it is a start. Human beings need about 5 kg/day in food, water, and oxygen, food accounting for about half the total. Unless you have regenerative life support you will need to carry it all with you. This does have the advantage of simplicity - we know how to do it, which is not the case for regenerative life support. For missions up to a few months the mass penalty is not excessive; 200 days' provisions and supplies come to about a ton per person, requiring about 3 cubic meters of storage space.

With storerooms, equipment bays, and assorted plumbing, our hab compartment for deep space transports may thus have a volume around 20 cubic meters per person. This in turn equates to about a ton or two per person for the basic hab pressure vessel, plus another ton or two of fittings and equipment, and for a 6 month mission a ton of consumables. So, altogether, each person (passenger or crew) carried by a transport class ship accounts for 3-5 tons of payload capacity.

In the rocketpunk era the standard way to reduce this was to carry passengers in Cold Sleep. But at our current level of knowledge this is magitech. We haven't a clue how to drastically slow down human metabolism, or even produce mere hibernation, let alone how to do it safely.

Another rocketpunk era classic was regenerative life support, those famous hydroponics tanks somewhere aft/below decks. Details were sometimes vivid, occasionally charming (fresh flowers in the wardroom of PRS Aes Triplex), rarely quantitative.

There is a rule of thumb that it takes 10 kg of food source biomass to support 1 kg of whatever is eating it: thus, for a purely vegetarian diet, about 0.75 tons of plant biomass per person. Biochemistry conveniently sees to it that the plants we eat also replenish our oxygen.

On this basis the break even point for regenerative life support is about 150 days. A 1953 (!) source cited at Atomic Rockets says 145 days. But this ignores the penalty mass of the hydroponics tanks. (Or aeroponics, whateva.) Greenhouse hydroponics on Earth seems to achieve yields of about 30-35 kg per square meter. One source mentions a 4 month growing season, suggesting that with year round operation we might do three times better, approaching 100 kg/m2.

Since we eat about a ton of food per year, this corresponds to about 10 square meters of growing surface per person. With access and working space perhaps 20 cubic meters per person - comparable, that is, to our estimate for living space, and probably with a comparable mass, about 3-5 tons per person, including a ton or so of biomass.

The structure and equipment mass needed to grow food shifts the tradeoff point: For missions less than about 2 years, the mass of stores plus storeroom capacity is less than the mass of a regenerative system. Most transport class ships - which for this purpose includes most military craft - are used for shorter missions that that, so they will dispense with the extra bulk and mass of full regenerative life support. They might have a garden deck, more for human factors than for its modest life support contribution.

There's also the little detail that we don't yet know how to do regenerative life support. This is one type of space research that can be done on Earth, and as space research goes it does not require a lot of expensive hardware. The Biosphere 2 fiasco doesn't prove a lot in and of itself, given the possible flake factor, but building a human supporting ecohab cannot be easy, or someone would have done it by now.

But we won't really need regenerative life support until we are establishing long term stations or bases. And my guess is that we'll learn the techniques gradually, in the process of reducing dependence on costly supplies from Earth. Regenerative life support is, on some level, a sophisticated form of gardening, and gardening has always called for patience.

Two other life support considerations: Radiation, and heat.

Without shielding, the cosmic radiation dose in deep space is about 400-900 millisieverts per year, where 1 Sievert = 100 old fashioned rems. The current career limit is 4 Sieverts for astronauts, 2 Sieverts and change for nuclear industry workers. Thus for long term habitation we will need enough shielding to diminish the penetrating radiation by about tenfold. This requires about 100 grams per square centimeter - a ton per square meter. And this shielding has to be applied all around, because cosmic rays can come at you from any direction.

This is not a problem for big permanent habs, but it is far too massive for transport class ships. This is one more reason to favor fast orbits for human travel. A ship's habitat might provide enough shielding to cut radiation by half, so that a 3 month tranfer mission provides about the same radiation exposure as a year living aboard a shielded hab.

And don't forget plain old heat management. An object at room temperature radiates about 400 Watts per square meter, which you will have to replace - but at 1 AU you are also exposed to a solar flux of 1400 W/m2 on surfaces directly facing the Sun. Managing heat, both from onboard power and solar flux, to keep the hab in the human comfort zone will be a constant task.

In fact, all of life support will be a constant task. In rocketpunk days maintaining the life support system was treated as an afterthought to the cool space stuff like astrogation and engineering. This is unlikely to be the case.

This being All Hallows' Eve, I'll just leave you with this thought for your contemplation: Cascading life support malfunction.

Related post: Spaceship Design 101.

Monday, October 26, 2009

History, From Above

ISS Above Ionian Sea
This image from Astronomy Picture of the Day - see it here in magnificent full size - gives a whole new meaning to 'overview of history.' Seen from the Shuttle Endeavor, the International Space Station passes above Sicily and heads east over the Ionian Sea, with the instep and heel of the Italian boot seen just to its left.

The Ionian Islands are the group above and to the right of the ISS, just off the coast of Greece. One of them is Odysseus' Ithaca (though it is uncertain whether his home was the same as modern Thiaki). The site of Troy is also hazily visible, straight up from the Ionian Islands about two thirds of the way to the limb of the Earth. The ISS will cover this distance in about 100 seconds, some three million times faster than Odysseus' trip home.

History's three greatest galley battles all took place within this field of view. Salamis (480 BC) is only hazily visible on the Aegean coast of Greece, but Actium (33 BC) was fought just off the lagoon to the left of the Ionian Islands, and Lepanto (1571) in the gulf just behind them.

In fact these waters are the Belgium of Mediterranean naval history: A disproportionate number of seafights have taken place here over the centuries, from Corcyra, the opening round of the Pelopponesian War in 31 BC, to Navarino (1827), the last fleet battle under sail, and Cape Matapan (1941). Also visible here is Taranto, Italy, where the British staged the first successful air raid against an anchored fleet more than a year before Pearl Harbor.

And in the time it has taken you to read this, the ISS will already be far to the east, gliding across the skies above the Silk Road.

Wednesday, October 21, 2009

Spaceship Design 101

Discovery (from 2001, the novel)
A lot of us would like some system for designing spaceships, at least in outline, for use in games, detailed fictional settings or physical or virtual 3D modeling.

The procedure I have seen most often begin by defining a hull. This gives you the main dimensions of the spacecraft, its surface area and volume capacity, perhaps along with constraints such as maximum load and drive acceleration. This is a natural approach. I used it for my battleship-era warship specification sim, SpringStyle, and it is retained by its independent offspring, SpringSharp.

But for deep space craft it is seriously misleading. Ships and aircraft, says Captain Obvious, move through a fluid medium that shapes and constrains their design. Deep space craft do not. Their overall design constraints are more architectural: supporting the craft against its own thrust, along with stresses from attitude change maneuvers, the thump of docking, thermal flexing, spin loads, and the various other kinds of abuse that spacecraft are subject to.

This is as good a time as any to point you to the Atomic Rockets pages on basic and advanced design.

I will argue that deep space craft have essentially two sections that can largely be treated separately from one another. One section is the propulsion bus - drive engine, reactor if any, solar wings or radiator fins, propellant tankage, and a keel structure to hold it all together. The other is the payload section that it pushes along from world to world.

There are both conceptual and economic reasons to treat them separately. Conceptually, because a propulsion bus might push many different payloads for different missions, such as light payloads on fast orbits versus heavy payloads on slow orbits. A little noticed but important feature of deep space craft is that you cannot overload them. They do not sink, or crash at the end of the runway, or even bottom out their suspension. They merely perform more sluggishly, with reduced acceleration and (for a given propellant supply) less delta v.

A very large station or hab might well have a modified ship drive as its main stationkeeping thruster. Or it may rely on a ship coupling to it, as the ISS is shunted by Soyuz craft docked to it.

Conceptual logic is also economic logic. The outfits that build drive buses would like to sell them to lots of different customers for a broad range of assignments.

This is not necessarily an argument for true modular construction, with drive buses hitching up to payloads on an ad hoc basis like big-rig trucks and trailers. Building things to couple and uncouple adds complexity, mass, and cost - plug connectors, docking collars, and so forth. Moreover, drive buses intended for manned ships need to be human-rated, not just with higher safety factors but provision for supplying housekeeping power to the hab, etc. But these things, along with differing sizes or number of propellant tanks, and so forth, can all be minor variations in a drive bus design family.

The payload we are most interested in is, naturally, us. The main habitat section of a deep space ship closely resembles a space station. It is likely that habs intended for prolonged missions will be spun, for health, efficiency, and all round convenience. (Flush!) The design of a spin hab is dominated by the spin structure and - unless you spin the entire ship - the coupling between the spin and nonspin sections.

Because ships' spin habs have the features of stations they may be used as stations, and again we can imagine design families, with some variants intended for ships and others as orbital platforms having only stationkeeping propulsion. Habs are the one major part of a deep space ship that correspond fairly well to our concept of a hull. Spin habs are entirely different in shape, but the shape is constrained; once you build it you can't easily modify it, beyond adding another complete spin section.

Pause to question another familiar convention here. Since at least Heinlein days spinning ships have typically been given a control room located on the spin axis, and perhaps nonspinning, where the astrogators can use their instruments unhampered. But isn't this equivalent to the circular astrogation slide rule? The navigators will do their normal work on monitors. In the inevitable space emergency there will no doubt be coelostats available, or other workarounds. But there's no reason not to locate the ship's main operating control room in the spin section, closer to the people who work there.

Though I'd be happy to be persuaded otherwise. I have always liked Heinlein's penthouse style control rooms at the forward/top end of the ship (plus the fact that he never called it a bridge). If Hollywood came calling I'd bend realism here in a nanosecond, not least because a 'top' control station is visually easy to understand, a sort of Aha! moment for viewers. But I suspect it is a minor cheat.

For those with bank cards at the ready, buying a deep space ship might be not unlike buying a computer. If your mission needs are fairly standard, you check off options on a menu. Those with more specialized requirements can select major components - perhaps a drive bus from one manufacturer, a main crew hab from another, along with custom payload sections, service bays, and so forth, assembled to your specifications.

In fact, both technology and probable historical development suggest that fabrication and overall assembly will be two distinct phases, carried on in different places, quite unlike either shipyard or aircraft assembly practice. In the early days, large deep space craft will be built the way the ISS was, assembled on orbit out of modules built on Earth and launched as payloads. In time fabrication may move to the Moon, or wherever else, but final assembly (at least of larger craft) will continue to be done at orbital facilities. I call them cageworks, on the assumption that a cage or cradle structure provides handy anchoring points for equipment.

For game or sim purposes, my advice would be to treat drive buses and hab sections as the primary building blocks for ships, whether these components are permanently attached to each other or simply coupled together. Both approaches might be in use.

A couple of provisos. All of the above applies mainly to deep space craft, especially with high specific impulse drives. Ships for landing on airless planets have some similar features. Ships that use rapid aerobraking, however, are aerospace craft and broadly resemble airplanes, even if they never land or even go below orbital speed.

And I have said nothing of warcraft. Kinetics are essentially just another payload. Lasers, and other energy weapons such as coilguns, probably draw power from the drive reactor, calling for some modifications in the drive bus. These things don't much affect the overall configuration. Armor protection would, but discussions here have left me doubtful of its value against either lasers or kinetics. Laser stars and other major warcraft may not be dramatically different in appearance from civil craft of similar size.

Monday, October 19, 2009

Thirty Planets

Extrasolar Planet
That is how many new discoveries are being reported at the Extrasolar Planets Encyclopedia website, along with three new brown dwarfs.

I was tipped off by a political blog, which linked to this CNN article. Yes, the article says 32 planets, but I'll go with the Paris Observatory. It may be a matter of updated information, since the observatory site links an email that references 29 plus 1.

No details yet about the newly reported planets - the image above, from the CNN site, seems to show a planet orbiting a double star, but it may just be a generic exoplanet from file footage, so to speak.

But thirty new planets (at least!) have swum into our ken. Wow.

Related posts: The California planet search team reported a haul of 28 planets in 2007.

Saturday, October 17, 2009

A Plume After All

L-CROSS Centaur Impact Plume
As it turns out, the L-CROSS mission did create a visible plume. Neener neener neener, says the Moon and the L-CROSS science team. I saw the news in the Los Angeles Times, dead tree edition, and it is online at Apparently the plume was imaged by a camera aboard L-CROSS itself, just not the imagery everyone was watching in real time. Judging from the stark contrast, they had to tweak up the contrast value to see it.

According to Anthony Colaprete, head of the team, the plume brightness was 'at the low end of our predictions.' (Do'h - that's why you didn't give us our show!)

Because I think of ice crystals as being bright, this does not seem positive for water, but that is probably an extremely naive interpretation. Much more to the point, the L-CROSS team evidently got plenty of good data, and over the next few weeks we may start to hear what they are learning from it.

Thursday, October 15, 2009

The Time Scale of Space II - Travel

A couple of posts back I looked at the historical time scale of space. Now let's look at the human time scale, the pace of travel.

We know how long this takes with current technology: about 9 months to Mars, and in the example of Cassini, three years to its Jupiter flyby and seven years to Saturn. Happily we do not have the choice of chemfuel or waiting for magic; the Dawn mission to Ceres and Vesta is already using that classic SF standby, ion drive. This is not suitable for large, human-carrying spacecraft, but other electric drives are.

Drive details matter less than the drive's power output, because for deep space travel with Realistic [TM] high specific impulse drives we must be concerned not only with speed (technically, delta v) but also acceleration. High specific impulse drives require enormous power in relation to thrust, and a top speed of 100 km/s will not get you to Mars quickly if it takes you a year to build up to it.

My convenient figure of merit for drive power density is one kilowatt per kilogram - on the same order as gasoline engines (and about 10x better than present day shipboard nuclear power plants.) At this power density, a drive with an exhaust velocity of 50 km/s has a thrust/mass ratio of 0.004, meaning it can just push itself along at 4 milligees. Attached to a ship, it might might waft it forward at 1 milligee or so.

By the bone crushing standards of SF acceleration - or actual Earth liftoff - this is feeble stuff, less than freight train acceleration. But keep it up for a month and you're booking along at 25 km/s - well above solar escape velocity for a tangential burn departing Earth's orbit.

Here are outline characteristics for a small ship, with a 100 megawatt power plant and full load mass of 750 tons, half of it propellant:

Drive engine, 100 tons
Tankage and structure, 75 tons
Payload section, 200 tons

Propellant, 375 tons

Given an exhaust velocity of 50 km/s this ship has a mission delta v of 34.7 km/s. It burns off propellant at 80 grams/second for a total burn time of 4.375 million seconds, 51 days, giving it an average acceleration of 0.81 milligees. It can reach Mars in about three months - its delta v is sufficient for a two month orbit, but the prolonged burns will add another month; in fact, the ship is under power for more than half the trip.

Replace the payload section with a much larger one, 750 tons, and mission delta v falls to 17.0 km/s, just enough for the Hohmann transfer to Mars, plus the (inefficient) spirals a low-thrust ship must use to enter and leave a planetary gravity well. This is good enough for slow freight, which in a thriving space economy will be the great majority of traffic.

All of these details are pretty arbitrary, except for the important ones, the basic relationships of drive power output, acceleration, and specific impulse that determine how fast you can get wherever you are trying to go.

Our concern is with passenger traffic in the broad sense, human travel, and for that we want fast orbits. Orbit calculations are far above my math pay grade. Happily the Atomic Rockets site has a wealth of information plus some handy links. For those who want to play along at home, this online calculator will give you the orbit parameters, delta v requirement, and travel time for orbits ranging from the economical Hohmann transfer to semi-fast orbits at just below solar escape speed. For faster orbits a flat space approximation starts to give decent results.

For travel in the inner Solar System, at least out to Mars, I am partial to solar electric drive. It has about as good a prospect as nuke electric does of hitting the 1 kW/kg benchmark, and it has the enormous virtue of having practically no moving parts. Whereas a nuclear electric plant is the ultimate steampunk maintenance nightmare, a steam engine in space.

The only problem with solar electric is that it gasps for light beyond the orbit of Mars. Sunlight at Ceres has only a seventh of its intensity at Earth, so a drive good for 1 km/s per day at 1 AU now takes a week to put on 1 km/s. A trip that might take 6 months by nuclear electric drive might take 9 months by solar electric due to sluggish performance in the asteroid belt.

For outer system exploration a VASIMR style variable specific impulse drive also becomes handy, and is probably not too difficult to achieve. If, for a given drive power output, you double the specific impulse and halve your thrust and thus acceleration, your total power requirement is (ideally) unchanged, but total delta v is doubled, while your propellant consumption falls by a factor of four.

The result? With a VASIMR type drive, travel time increases not in direct proportion to distance, but as the 2/3 root of distance. Suitably tuned, the drive outlined here reaches the main asteroid belt in about 6 months, Jupiter in a year, Saturn in a year and a half, Neptune in three years, and Eris, beyond the Kuiper Belt near 90 AU, in about 7 years. (These are careful guesses, not worked out orbits!)

In rocketpunk days they did not blink at multi-year journeys, and you could say that the true first orbital mission was Magellan's, three years to go once around. We can explore Jupiter and Saturn; human missions to the outer planets and beyond are problematic, at this techlevel, on human factors grounds.

I've suggested that a benchmark for 'routine' travel is about three months, experience with submarines showing that being cooped in a can becomes progressively difficult beyond this time. Even aboard luxury liners, shipboard romances start getting complicated, and threats to the piano player get serious. Oh yes, also the little detail of radiation - the longer your travel time, the more shielding you need, meaning penalty mass.

This doesn't mean that we can't go to the asteroid belt, especially if it turns out to be full of Valuable Asteroid Stuff; it just means that cabin fever becomes a challenge. (As does radiation shielding.)

For the bloodthirsty among my readers, which is most of you, note that warlike expeditions will tend to follow slower orbits than civil passenger transports, because they had better carry delta v for a round trip, or least an abort to a friendly base. Drop tanks won't really speed you up, because their mass reduces acceleration, making transfer burns more sluggish. For a faster military trip you'll have to revert to staging, and ditch power plants as well as tanks.

Faster travel would be helpful - for peaceful as well as warlike purposes - but speeding things up will be surprisingly difficult. For faster orbits we must increase not only peak speed but acceleration; in fact, for brachistochrone and semi-brachistochrone orbits the required acceleration goes up as the square of peak speed. (To make the trip in half the time you must go twice as fast in half the time, calling for four times the acceleration.)

Halving travel time - three months to Ceres, six months to Jupiter - thus requires an eightfold increase in drive power density, into the same range as jet engines, several kilowatts per kilogram. For this we will probably need a drive that generates its power directly, rather than requiring a separate power plant. Fusion drive is the classic if speculative example, though there are alternatives.

So what does all of this mean? For some period in the midfuture, perhaps a lengthy one, the pace of travel will be more or less as outlined here - about three months to Mars, six to Ceres and other points in the main asteroid belt, a year to Jupiter. Coming next, a look at the social and political implications of these travel times.

Related post: Last year I wrote a bit on getting around the Solar System, under much the same tech assumptions I've described here.

Friday, October 9, 2009

Dark Flash

There is a story, no doubt apocryphal, about a cub reporter sent on his first assignment to cover a society wedding. He came back to tell his editor that there was no story because the groom never showed up.

This morning's L-CROSS mission press conference, which I just got done watching on NASA television, had a bit of that flavor. The underlying interest in L-CROSS centers on the search for lunar ice. But the short term payoff - the Hollywood money shot - was the bright plume that the Centaur stage impact was supposed to kick up from the lunar surface, expected to be visible from Earth through observatory telescopes.

No plume was visible, at least not in the early results. And it was clear at the press conference that neither the mission team nor the assembled media knew quite what to make of it. The mission team put on its dog and pony show, without the pony - even showing video footage that showed nothing but the shadowed floor of Cabeus crater. The media seemed just a bit annoyed at not getting their expected show. Not until near the end of the question period did anyone ask the question on my mind: What does it tell us that we didn't see the expected plume?

Apart from the missing - or unexpectedly dim - plume, the impact clearly yielded some 'interesting' data. Whatever happened to the debris plume, spectrometers took post-impact spectra that were clearly different from pre-impact spectra. A small (barely more than one pixel, about 20 meters) but conspicuous impact crater was imaged by L-CROSS just before it too impacted, and the mission team noted something else they evidently did not expect, a sodium flash.

Did the spectra also have any hint of water? (Or not?) The mission team was cagey about that, as you'd expect they would be unless Cabeus had erupted like a broken fire hydrant. When the head guy (I missed his name) said he hadn't yet examined the spectra for hints of water, a reporter drew laughs by saying 'Oh, come on!' But the mission scientist kept his poker face, and I can't guess whether he did or didn't see water indicators.

Perhaps most likely it is genuinely too early to tell. They saw something, including indicators of sodium, so conspicuous (and in a sense so uncontroversial) they could mention it even in this early going. But it will take analysis of the results to know what else they did or didn't find.

The one thing clear is that L-CROSS delivered a surprise, a dog that did not bark in the night.

Tuesday, October 6, 2009

The Time Scale of Space

Interplanetary Spacecraft
When is the space future supposed to happen? The question is raised by comments on the last couple of posts, especially some of Jean Remy's observations. Among space minded people today a sense of frustration and stagnation is pervasive. But this is largely a result of distorted historical perspective.

We entered space with a spectacular splash, due to particular circumstances, AKA the Space Race. Rather than working up gradually to a lunar expedition by building an orbital station first, as expected in the 1950s, we took it in one straight shot, then woke up with moon rocks in our pockets and a great big hangover. We did what? We went where?

Just as unexpectedly, satellites took over most of the jobs that the 1950s assigned to a space station. Instead of having immediate (and valuable/profitable) tasks, such as weather observation and telecoms relay, a space station is needed only for long term development. The same can be said of human spaceflight itself. Robotic spacecraft serve our current practical needs very well, and they are carrying out our first reconnaissance of the Solar System faster than anyone in the 1950s dreamed. We are not sending people up to watch hurricanes, we are sending them up to learn how to live and work in space.

True, there is a short-term crisis in US human spaceflight - our current architecture is nearing retirement, and its replacement is over budget and behind schedule at best, at worst a major Washington boondoggle. This is a temporary and parochial concern. Abandoning a reusable shuttle in favor of an 'old fashioned' capsule also feels like retrogression, but it merely accepts a reality: The Shuttle, like the Great Eastern, was too much too soon. The technology is not mature enough for an efficiently reusable vehicle, and the traffic does justify one.

Meanwhile people like Burt Rutan are experimenting with lower cost approaches to launch and initial ascent. Whether or not suborbital tourism succeeds as a business, and it might, this work will pay off when the launch market reaches the point of demanding reusable craft.

So what might we expect from here?

The current year round population in space is six. Suppose it were to grow, through the usual fits and starts, at an average 4 percent per year. First we'll try this out on the past. Run backward from 2009, this growth rate gives us three people in space in 1991 - about when Mir entered full service - and one person in 1963, soon after we started traveling in space at all. As predictions of the past go this is imperfect but not bad.

Now let's apply it to the future. Over the next few decades, growth is glacially slow:

2025: 11 people
2050: 30
2075: 80

So two generations from now there are still only 80 people living in space - barely enough for a robust orbital station and minimal outposts on the Moon and Mars. The space population passes 100 in 2081, and by 2101 there are a shade over 220 people in space - just enough, perhaps, to support the sort of travel infrastructure that the movie 2001 pictured for a century earlier.

But in the 22nd century, compound arithmetic starts to kick in:

2125: 568
2150: 1513
2175: 4034
2200: 10,753

More than ten thousand people in space is a lot. At this point the human Solar System begins to resemble our familiar image - large orbital stations; regular scheduled passenger service at least to the Moon and perhaps to Mars; space based industries - surely propellant production, likely some mining and fabrication as well.

Continue the same growth rate until 2300 and there are more than half a million people in space, the equivalent of a medium sized city and suggestive of at least incipient colonization.

Any compounding formula, carried far enough, becomes absurd. This one gives 1.4 billion people in space in 2500 and 3.5 trillion in 2700. In real life such trends bump up against something. Short of magitech on the one hand or catastrophe on the other, living in space will remain more difficult and costly than on Earth, so the population will probably settle in at some modest fraction of Earth's population - which might mean anything from a few hundred people to a few million.

But the real point is that the time scale examined here is not so different from the classic time scale of the rocketpunk era. Heinlein, after his early (and wildly space-optimistic) Future History, got cagey about specific dates. But the interplanetary futures of Space Cadet or The Rolling Stones generally seem to be set around the 22nd century, and much the same for Clarke's hard SF, aside from books directly linked to 2001. The scale of things was closer to what my formula predicts for the 23rd century. But a few decades this way or that, or even 100 years, is nearly a quibble on the time scale of centuries.

The truth is that human expansion into the Solar System was always going to be gradual, because the Solar System is so big. 'Murricans should have been particular aware of this, because of the prolonged colonial prelude to our national experience. It was 115 years from Columbus to Jamestown, and another 168 years to the the Declaration of Independence. On the same time scale, starting from 1969, we might expect the founding of Luna Base in 2084 and the inevitable Revolt of the Colonies in 2252.

Viewed in large historical perspective our space progress is just about on schedule.

Image of proposed HOPE Callisto mission ship via Atomic Rockets.

Related posts: I discussed our rate of progress on the 39th anniversary of Apollo.

Sunday, October 4, 2009

Some Sunday Afternoon Reading

A couple of links I have come across that relate to the last few posts on space access and space costs.

When Rocket Science Meets the Dismal Science. An analysis of orbit lift costs that uses much the same method I do, and reaches a similar result. (Which is reassuring!) In a nutshell, reusable orbiters are bigger, more complicated, more cantankerous, and therefore more expensive to build and fly than expendables are. Therefore you need a lot of traffic before they pay off.

Space Cynics. This blog isn't cynical about space itself, but it is a bit cynical - and not without justification - about what they call the 'alt-space' movement and (would-be) industry. Not guys like Rutan, who has a track record, but too many people who are fueled by technological and economic wishful thinking.

I have been harping on this subject a lot lately because it is so central to the enterprise of space. Things have not happened nearly as quickly as we expected. But the space community has been more evangelical than analytical about it. Somewhat meta, but how much have we really thought about the human presence in space since the rocketpunk era?

Back to links, and now on the nuts & bolts side, Space Launch Report is a handy site for finding out what is actually going up there. I was looking for year-by-year launch totals and found them here, at least back to 1998, along with descriptions of recent launches and a nice series on the Thor IRBM and its evolution into the Delta space booster.